1. Field of the Invention
The present invention generally relates to a method and apparatus that maintains devices at the same temperature on a silicon on insulator (SOI) substrate.
2. Description of the Related Art
Conventional systems utilize identical devices (transistors and diodes) in different circuits at different locations on a chip. These devices dissipate different amounts of heat depending upon the duty cycle of the circuit and surrounding circuits and devices. Further, these devices operate at different temperatures. Thus, each device, even if designed identically, may have different current/voltage (IV) characteristics so the circuits will not behave identically, which can result in design flaws.
The temperature effects of NMOS and PMOS devices are well known. For example, the mobility of the carriers is inversely proportional to the temperature. The mobility may decrease (and resistivity may increase) by about 40% for each 100.degree. C. temperature rise. Further, both drain current and threshold voltage decrease with mobility. Accordingly, devices at different temperatures can have very different characteristics.
To minimize these problems, devices that need to be matched are placed in close proximity to each other. However, this is not always possible and therefore cooling may be applied to the chip using external heat sinking. Even this is not completely effective, especially in low voltage regimes. Devices fabricated in silicon on insulator (SOI) technology presents an additional problem because they are thermally isolated by the insulator which makes bulk cooling not as effective.